JCV, a human polyma virus, causes the fatal demyelinating disease progressive multifocal leukoencephalopathy. The virus has an extremely restricted host range and grows well only in human fetal glial cells. The two 98 bp tandem repeats located upstream of the origin of replication in the JC genome have been cloned into the pCAT3M plasmid, which has the bacterial choloramphenicol acetyl transferase gene. The JC tandem repeats, which share significant homology with an 82 nucleotide rat brain specific RNA sequence, were shown to contain stronger enhancer-like activity in fetal glial cells than in other cell types. The narrow host range of JCV is at least partly the result of restricted enhancer activity.